Replace just the diff part in '03 Tochigi-Fuji rear?
 

I have an '03 that originally came with the 3.9 Tamagotchi rear end. When changing the fluid in the rear, some metal pieces came out. Apparently parts of the clutch pack or some such. Big chunks of metal are usually put in the diffs because they have a function, so them coming out with the fluid appears less than perfect. MT.net to the rescue! I bought a 4.1 Torsen from another member on this forum and plopped that in. So far so good. Unfortunately that Torsen has developed what appears to be a hairline crack in the diff housing (not the carrier) and piddles tranny fluid all over my shiny exhaust and in general stinks and messes the place up.

I'm picking up rear-end #3 for this car tomorrow to get it going, but for the long term I think one spare rear end in the garage should suffice.....

Question: Can I make a hybrid? I believe it should be possible. Take the housing, as well as the 3.9 ring and pinion gear from the '03 rear end and just replace the insidy-bits with the diff out of the Torsen.
Any problems to be expected?

It should work.

You'll want to look up the procedure for setting gear mesh/depth in the service manual.

I do have that and already looked it over- shouldn't be too bad.

Same as installing any other LSD. You shouldn't even need to set pinion depth since you aren't removing the pinion from the 3.9 housing - just take the T-F out of the 3.9, put the ring gear on the Torsen LSD, drop it in, and set backlash.

I can do that- thanks!